1. Department of Information Engineering, Computer Science and Mathematics, University of L’Aquila, Via Vetoio, 67100 L’Aquila, Italy
2. Department of Mathematical Sciences “Giuseppe Luigi Lagrange”, Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129 Turin, Italy
3. National Institute for Nuclear Physics, Sezione di Torino, Via Pietro Giuria 1, 10125 Turin, Italy